This invention relates to an interferometric signal processing apparatus.
Interferometric signal processing apparatuses use an interferometric technique to minimize the noise contributions of the signal processing elements acting on the input signals, so that finer details of the differences between the input signals can be analyzed with less limitation. Carrier suppression is used to increase the sensitivity of interferometric signal processing apparatuses.
Interferometric signal processing apparatuses have been used in two configurations.
The first configuration is described by A. L. Lance, W. D. Seal and F. Labaar at page 273 in xe2x80x98Infrared and Millimeter Wavesxe2x80x99, Vol 11, 1984, published by Academic Press, Inc. The interferometer described consists of a bridge in which a delay line forms one arm of the bridge and a phase shifter and an attenuator form the other arm of the bridge. The two arms of the bridge are then combined to achieve carrier suppression. Both arms of the bridge are fed with a signal from a unit-under-test.
The interferometer described by Lance et al measures FM noise in the unit-under-test. The delay line acts to introduce a differential group delay between the signals present at the power combiner, so that if the frequency of the signal produced by the unit-under-test varies with time, the signals appearing at the power combiner will have different frequencies. The output from the power combiner will be in accordance with the phase difference between the signals.
The sensitivity of the interferometer described by Lance et al is proportional to the differential group delay created by the delay line. Consequently, a significant group delay is considered desirable.
The second configuration is described by E. Ivanov in a paper titled xe2x80x98A new mechanism for paramagnetic back-action effect in a gravitational wave antenna with a microwave cavity transducerxe2x80x99 at pages 1737-1742 in J. Phys. D: Appl. Phys, Vol 28, 1995. The interferometer described consists of a bridge in which a microwave transducer, operating as a displacement sensor for a Nb gravity-wave detector, is radiatively coupled into one arm of the bridge via a pair of microwave antennae and a circulator, and the other arm of the bridge has a phase shifter and an attenuator. The two arms of the bridge are combined to achieve carrier suppression. Both arms of the bridge are fed with a signal from a microwave signal source.
The interferometer described by Ivanov is used to increase the sensitivity of the measurements from the transducer by utilising carrier suppression. The phase shifter in the other arm of the bridge is used to ensure that the signals appearing at the power combiner are 180xc2x0 out of phase to achieve carrier suppression.
Sensitivity is of primary importance in measuring displacement of the Nb gravity-wave bar. Transducers with a high Q are used to increase the sensitivity of the measurements, however such high Q transducers introduce a significant group delay into the one arm of the bridge. In turn, this results in a significant differential group delay in the bridge of the interferometer described by Ivanov, with differential group delays of 10 xcexcs or more at 10 GHz.
The significant differential group delay in the bridge places stringent requirements on the stability of the microwave signal source used. If the frequency of the microwave signal source varies with time, the output from the power combiner will include signals which are due to noise of the source, including its PM, FM and AM noise. This can significantly limit the sensitivity of the displacement measurements.
In both of the above configurations, the device in the one arm of the bridge acts as a frequency discriminating element and alters the relative phase shift of the signal travelling through it depending on the frequency of the signal.
The sensitivity of the interferometer described by Lance et al is directly proportional to the group delay of the delay line.
Similarly, in the interferometer described by Ivanov the sensitivity of the transducer is proportional to its Q factor. Consequently the sensitivity of the transducer is proportional to the group delay of the transducer, since the group delay of the transducer is substantially equal to the Q factor of the transducer divided by the carrier frequency.
The interferometric signal processing apparatus of this invention produces an output selectively containing information about the amplitude, phase or both amplitude and phase noise produced by a device-under-test (DUT). The invention seeks increase the frequency bandwidth over which the apparatus is useable and to reduce the limitations set by the noise contributions of the signal source by minimising the differential group delay.
According to a first aspect of this invention, there is provided an interferometric signal processing apparatus producing an output signal from a first input signal and a second input signal, said input signals having substantially equal carrier frequencies, comprising:
a bridge having a first arm and a second arm, each arm having a first end and a second end, the first and second input signals being input to the first end of the first and second arms, respectively;
a device-under-test provided the first arm;
a carrier suppression means connected to the second ends of the first and second arms to produce a carrier-suppressed signal at its output;
an amplifier arranged to amplify said carrier-suppressed signal; and
a mixing means responsive to the amplified carrier-suppressed signal and a carrier-dominated signal to produce the output signal;
wherein the differential group delay between:
the first end of the first arm and the output of the carrier suppression means; and
the first end of the second arm and the output of the carrier suppression means
is less than or equal to 1000/fo, seconds, where fo is the time-averaged mean value of the carrier frequencies of the input signals.
In the previous statement and throughout this specification, the term xe2x80x98device-under-testxe2x80x99 is intended to mean anything that is responsive to a signal at the carrier frequency, and may include a number of components or a circuit network.
Preferably, a delay means provided in at least one arm of the bridge to reduce the differential group delay.
Preferably, the carrier suppression means comprises a power combiner, a phase shift means and an amplitude matching means, the phase shift means and the amplitude matching means being arranged such that the power combiner produces the carrier-suppressed signal from signals input thereto.
In one arrangement, said carrier-dominated signal is a further signal produced by the power combiner.
In an alternative arrangement, said carrier-dominated signal is one of said input signals.
In a further alternative arrangement, said carrier-dominated signal is a third signal having substantially the same carrier frequency as the first and second input signals.
Preferably, said output signal is used in a control system, a feedback system or a read-out system.
Preferably, the apparatus further comprises a plurality of mixing means to produce a plurality of output signals.
By altering the phase difference between the carrier-suppressed signal and the carrier-dominated signal, it is possible to control whether the output signal corresponds to the amplitude of, the phase difference between, or both the amplitude of and the phase difference between the input signals.
For example, if the carrier-suppressed signal and the carrier-dominated signal are in phase (ie. a phase difference of 0xc2x0) when input to the mixing means, the output signal will correspond to the amplitude of the input signals. Throughout the specification, this arrangement is referred to as an xe2x80x98amplitude sensitive modexe2x80x99.
Further, if the carrier-suppressed signal and the carrier-dominated signal are in quadrature (ie. a phase difference of 90xc2x0) when input to the mixing means, the output signal will correspond to the phase difference between the input signals. Throughout the specification, this arrangement is referred to as a xe2x80x98phase sensitive modexe2x80x99.
Furthermore, if there is a phase difference of between 0xc2x0 and 90xc2x0 between the carrier-suppressed signal and the carrier-dominated signal when input to the mixer, the output signal will correspond to both the amplitude of and the phase difference between the input signals in proportions corresponding to the cosine and sine of the phase difference, respectively.
Advantageously, if two mixing means are utilized in the apparatus and are arranged so that one of said means is set to a phase sensitive mode and the other said means is set to an amplitude sensitive mode, the apparatus can simultaneously measure both phase and amplitude changes induced by the device-under-test. This arrangement is particularly advantageous in that the outputs of the two mixers contain all of the amplitude and phase information needed to fully characterize the properties of the device-under-test.
A further advantage of the apparatus is that it provides increased immunity to amplitude and phase noise in the signal source, nor is its sensitivity limited by the intrinsic noise of the mixers when compared with existing apparatuses.
Preferably, said apparatus has two mixing means, one arranged in an amplitude sensitive mode and the other arranged in a phase sensitive mode, to produce a pair of output signals corresponding to the amplitude and phase noise induced by the device-under-test. One or both of the output signals can then be used in a feed back control system in association with a voltage controlled attenuation means and/or a voltage controlled phase shift means to reduce one or both of the amplitude and phase noise in the device-under-test. In addition, one or both of the output signals can also be used in a further feed back control system to control operation of the phase shift means and the amplitude matching means of the carrier suppression means to maximize the carrier suppression, or to control certain characteristics of the device-under-test.